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Satellites Of Rogue Planets
Many discovered Rogue Planets are found to be hot giants. Some of them are hot enough to radiate enough heat and even light to support liquid water on a nearby satellite. Therefore, people wondered if such a satellite can be terraformed and if it can support life without the use of an Artificial Sun. Hot Rogue Planets Most of rogue planets detected are hot. Cold planets could only be detected if they pass in front of a star. Formation Rogue planets are formed in two ways: they can be ejected from a solar system by gravitational perturbations or they can form in a similar way to stars from collapsing of small molecular clouds. Some scientists argue that a molecular cloud must have a certain minimum mass to collapse, while others argue that, even if hydrogen and helium from the cloud cannot collapse, dust can form even smaller structures, down to the size of asteroids. A gas giant, formed from collapsing molecular clouds, should be hot enough to glow in visible light. Without fusion reaction, it will not be able to maintain its internal heat and will gradually cool. If satellites are formed in a similar way, they should also have a hot internal core. The question remains if the satellites would cool fast enough to have a solid crust suitable for terraforming before the planet cools too much to provide visible light up to the level requested for Earth-like plants. When a planet is ejected from its orbit, it undergoes tremendous tidal stress, which can break its crust. In case of a gas giant, the tidal forces can force hot gasses from inside to reach the 'surface' and make the planet glow. In case of small planets, tidal forces will create massive volcanism and lakes of lava. However, heat will soon dissipate and the planet will cool fast, probably even before leaving its home solar system. Possible moons It must be noted that when a planet is kicked out of orbit its moons are most often also kicked out of their orbits. So, ejected planets are unlikely to have moons. A planet formed in interstellar space will have a higher chance of having its own set of moons. On the other hand, free-floating small planets or asteroids can be captured in orbit by a rogue planet. This could allow moons old enough to be cold to orbit a hot rogue planet. Possible Heating One would agree that without a source of fusion rogue planets will cool down, but this process takes more then a human lifetime. It is possible to transform a cold rogue planet into an Artificial Sun. While some advanced civilizations would try to start nuclear fusion in the core, other civilizations would try something more easy, like smashing something into the planet. A hot rogue planet would provide heat for its moons, in a similar way Brown Dwarfs do. It might be possible that the radiated light could also be enough for plants to survive. It was also suggested that thermal heating could provide enough heat for a moon orbiting a rogue planet close enough. It is possible, but such a moon will have an extreme volcanism and will be uninhabitable. Another option is to build a source of heat and light on a smaller moon, that would work as an artificial sun, heating all terraformable moons. Conditions on moons The moons orbiting rogue planets could be very different from what we know. If we think about a planet pushed out of orbit, then most of its moons would had been ejected. Some close-orbiting moons might have survived, but, exposed to a powerful tidal stress, they might had became balls of liquid lava. After cooling, they would had changed their surface completely. Other moons might break apart, form rings and collide back. A moon formed from interstellar gas and dust would have higher concentrations of volatiles. Since the planet would cool fast, the moons would retain their volatiles for longer. While the planet is hot, its moons might behave like comets. Importance Most discovered rogue planets are hot giants. However, since these planets lack an internal source of heating, they will cool down. Scientists argue that there are far more cool rogue gas giants then hot ones in the Universe. Terraforming a moon of a hot free floating gas giant is a completely different concept then terraforming a planet orbiting a star.